1. Overview

This module is able to detect compressed body in received SIP message
and decompress it as well as compress the body for outgoing SIP
message. It works also for received HTTP request and replied HTTP
response (Kamailio cannot work in HTTP proxy mode).

The decision of whether to do compression or decompression is made
by detecting a special SIP header (default 'Content-Encoding') that
matches a given value - both header name and value can be set via
module parameters. If a SIP message is received with clear body and
you want to compress the body for outgoing, add the header in config
file. The header can be added to the local generated replies as well.

In other words, if the header is present in incoming SIP message, its
body is decompressed. If the header is present in outgoing SIP message,
its body is compressed. Therefore inside configuration file, the body
is in original format(e.g., plain text). In this way, the existing
functions to handle content of the body work as usual (e.g., to strip
codecs in sdp via sdpops or do substitutions via textops).

The functions used to compress and decompress are from zlib
library (http://zlib.net).

NOTE: for the moment the module cannot be used with topoh module,
overlapping in core event callbacks (will be fixed soon).

The immediate benefit of compressing the body is to reduce the size
of the SIP message, increasing the chances to stay under MTU for UDP
packets. From observation, the compressed body is in between 50% to 67%
smaller than the original size (e.g., a body of 431 bytes was
compressed to 230).

An use case can be when having peering traffic between two Kamailio
servers. Before relaying to the other Kamailio, use
in config file: append_hf("Content-Encoding: deflate\r\n").

2. Dependencies

2.1. Kamailio Modules

The following modules must be loaded before this module:

none.

2.2. External Libraries or Applications

The following libraries or applications must be installed before running
Kamailio with this module loaded:

zlib compression library (http://zlib.net).

3. Parameters

3.1. header_name (str)

Name of the header that indicates compression or decompression has
to be done.

Default value is "Content-Encoding".

Example 1.1. Set header_name parameter

...
modparam("gzcompress", "header_name", "Encoded")
...

3.2. header_value (str)

Value of the header that indicates compression or decompression has
to be done.

Default value is "deflate".

Example 1.2. Set header_value parameter

...
modparam("gzcompress", "header_value", "gzip")
...

3.3. sanity_checks (integer)

If set to 1, gzcompress module will bind to sanity module in order
to perform sanity checks over received SIP request. Default
sanity checks are done. It is useful to check if received request
is well formatted before proceeding to encoding/decoding.

Default value is 0 (do not bind to sanity module).

Example 1.3. Set sanity_checks parameter

...
modparam("gzcompress", "sanity_checks", 1)
...

4. Functions

None.

5. Config File

Next example shows how to enable compression for forwarded requests,
as well as replying with compressed body for HTTP requests. For SIP,
the request is received and forwarded to itself once, just for the
sake of showing a simple example.